complete threads till 06

exercises/07_threads/02_static/src/lib.rs

@@ -4,7 +4,10 @@
 use std::thread;
 
 pub fn sum(slice: &'static [i32]) -> i32 {
-    todo!()
+    let (s1, s2) = slice.split_at(slice.len() / 2);
+    let t1 = std::thread::spawn(|| s1.iter().sum::<i32>());
+    let t2 = std::thread::spawn(|| s2.iter().sum::<i32>());
+    t1.join().unwrap() + t2.join().unwrap()
 }
 
 #[cfg(test)]